Conversion of methanol feeds to aromatic compounds is an industrially valuable reaction. Conventional methods for converting methanol to aromatics can involve exposing a methanol-containing feed to a molecular sieve, such as ZSM-5. In addition to forming aromatic compounds, some olefins can also be produced. Reactions for conversion of methanol can be useful, for example, for creation of aromatics and olefins as individual products, or for formation of aromatic and olefin mixtures for use as naphtha boiling range or distillate boiling range fuels.
One difficulty with methods for conversion of methanol to aromatics is that the conversion reaction can have a relatively low yield of aromatics. The low yields from conventional methods can pose a variety of challenges, such as requiring large equipment footprints relative to total product volume as well as loss of initial reactant to various side reactions.
U.S. Pat. Nos. 4,049,573 and 4,088,706 disclose conversion of methanol to a hydrocarbon mixture rich in C2-C3 olefins and mononuclear aromatics, particularly p-xylene, by contacting the methanol at a temperature of 250-700° C. and a pressure of 0.2 to 30 atmospheres with a crystalline aluminosilicate zeolite catalyst which has a Constraint Index of 1-12 and which has been modified by the addition of an oxide of boron or magnesium either alone or in combination or in further combination with oxide of phosphorus. The above-identified disclosures are incorporated herein by reference.
Methanol can be converted to gasoline employing the MTG (methanol to gasoline) process. The MTG process is disclosed in the patent art, including, for example, U.S. Pat. Nos. 3,894,103; 3,894,104; 3,894,107; 4,035,430 and 4,058,576. U.S. Pat. No. 3,894,102 discloses the conversion of synthesis gas to gasoline. MTG processes provide a simple means of converting syngas to high-quality gasoline. The ZSM-5 catalyst used is highly selective to gasoline under methanol conversion conditions, and is not known to produce distillate range fuels, because the C10+ olefin precursors of the desired distillate are rapidly converted via hydrogen transfer to heavy polymethylaromatics and C4 to C8 isoparaffins under methanol conversion conditions.
Olefinic feedstocks can also be used for producing C5+ gasoline, diesel fuel, etc. In addition to the basic work derived from ZSM-5 type zeolite catalysts, a number of discoveries contributed to the development of the industrial process known as Mobil Olefins to Gasoline/Distillate (“MOGD”). This process has significance as a safe, environmentally acceptable technique for utilizing feedstocks that contain lower olefins, especially C2 to C5 alkenes. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C2 to C5 olefins alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al have also contributed improved processing techniques to the MOGD system, as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Conversion of lower olefins, especially propene and butenes, over ZSM-5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as liquid fuels, especially the C5+ aliphatic and aromatic hydrocarbons. Olefinic gasoline is produced in good yield by the MOGD process and may be recovered as a product or recycled to the reactor system for further conversion to distillate-range products. Operating details for typical MOGD units are disclosed in U.S. Pat. Nos. 4,445,031; 4,456,779, Owen et al, and U.S. Pat. No. 4,433,185, Tabak, incorporated herein by reference.
In addition to their use as shape selective oligomerization catalysts, the medium pore ZSM-5 type catalysts are useful for converting methanol and other lower aliphatic alcohols or corresponding ethers to olefins. Particular interest has been directed to a catalytic process (“MTO”) for converting low cost methanol to valuable hydrocarbons rich in ethene and C3+ alkenes. Various processes are described in U.S. Pat. No. 3,894,107 (Batter et al), U.S. Pat. No. 3,928,483 (Chang et al), U.S. Pat. No. 4,025,571 (Lago), U.S. Pat. No. 4,423,274 (Daviduk et al) and U.S. Pat. No. 4,433,189 (Young), incorporated herein by reference. It is generally known that the MTO process can be optimized to produce a major fraction of C2 to C4 olefins. Prior process proposals have included a separation section to recover ethene and other gases from by-product water and C5+ hydrocarbon liquids. The oligomerization process conditions which favor the production of C10 to C20 and higher aliphatics tend to convert only a small portion of ethene as compared to C3+ olefins.
The methanol to olefin process (MTO) operates at high temperature and near 30 psig in order to obtain efficient conversion of the methanol to olefins. These process conditions, however, produce an undesirable amount of aromatics and C2 olefins and require a large investment in plant equipment.
The olefins to gasoline and distillate process (MOGD) operates at moderate temperatures and elevated pressures to produce olefinic gasoline and distillate products. When the conventional MTO process effluent is used as a feed to the MOGD process, the aromatic hydrocarbons produced in the MTO unit are desirably separated and a relatively large volume of MTO product effluent has to be cooled and treated to separate a C2-light gas stream, which is unreactive, except for ethene which is reactive to only a small degree, in the MOGD reactor, and the remaining hydrocarbon stream has to be pressurized to the substantially higher pressure used in the MOGD reactor.
U.S. Pat. No. 3,998,898 describes a method for manufacture of gasoline using an MTG style process. In U.S. Pat. No. 3,998,898, a potential gasoline including aromatic compounds is manufactured from a feed that contains two types of aliphatic compounds. The feed can contain aliphatic compounds corresponding to a) “difficultly convertible” compounds, such as carboxylic acids and short chain aldehydes, and b) “easily convertible” compounds, such as aliphatic alcohols, ketones, and aldehydes containing 3 or more carbons, with the mixture having sufficient “easily convertible” compounds to make up for a stoichiometric deficiency due to the presence of any carboxylic acids in the feed. The use of a mixture of a “difficultly convertible” compound and an “easily convertible” compound meeting the specified criteria is described as improving the yield of gasoline boiling range compounds at the expense of compounds having 4 carbons or less.
U.S. Pat. No. 7,820,867 describes a variation on the methods from U.S. Pat. No. 3,998,898. The '867 patent describes integration of a reaction for converting synthesis gas to methanol (or other oxygenates) with a methanol to gasoline reaction. In the integrated system, the “difficultly convertible” compounds can be introduced into the reaction stage for conversion of synthesis gas to methanol. The same definition for “difficultly convertible” compounds used in U.S. Pat. No. 3,998,898 is maintained in the '867 patent.
Despite numerous prior art processes, there is an ongoing desire to improve methods of converting methanol to aromatics that yield a higher amount of aromatics than the prior art methods. There is a particular interest in methods that produce high yields of paraxylene, considering paraxylene's value in industry and its use in the manufacture of terephthalic acid, an intermediate in the production of synthetic fibers.